This invention relates to a guided wave optical polarizer of the type for use in a large number of optical fiber sensors and transmission systems, for example, fiber optic gyroscopes.
As is well known to those of ordinary skill in the art, at short wave-lengths, such as will be used in many sensor systems, integrated optical waveguides often suffer from optical damage. More specifically, it is generally known that propagation along the Z-axis eliminates this problem. On the other hand, undesired modal conversion properties restrict the application of such guides to those not involving electrical fields. The modal conversion properties are, as is well known, a result of the approximate degeneracy of the quasi TE and TM modes.
Thus, it is also known from the prior art that a discrete optical polarizer may be fabricated by a series of dielectric and metal overlays on an existing waveguide, for example, a lithium niobate or a lithium tantalate crystal having a light guiding region defined thereon by means of, for example, titanium diffusion. By using the Z-propagation direction certain important advantages are obtained.
Included among these advantages is the fact that the polarizer will have high resistance to optical damage, and the extinction ratio will therefore be independent of both input power and time. In addition, the mode profile will be reasonably symmetric due to equal diffusion constants for titanium and the two directions orthogonal to the propagation direction and thus, efficient coupling to optical fibers may be realized. And finally, modal conversion is eliminated due to the lifting of the degeneracy of the two orthogonal modes. This is due to a coupling of the TM-like modes to the surface plasma wave supported by the dielectric metal interface.
Several prior art approaches have involved the use of different materials. More specifically, the most common approach is to sputter or evaporate an SiO overlay onto the light guiding region and thereafter, depositing, by either sputtering or evaporation, a layer of aluminum over the dielectric SiO. While generally giving high extinction ratios, for example, as shown in FIG. 3 which is representative of a lithium niobate, titanium diffused guide, with a dielectric layer of SiO overlayed by aluminum, the use of the prior art known dielectrics has been highly thickness dependent with any variation in any direction, i.e. towards the thinner or thicker side, resulting in less than good results. More specifically, the peak for extinction ratios as shown for a waveguide polarizer with three nominal guide widths, shown for clarity in FIG. 3 occurs within a very narrow range, for example, within a buffer thickness of 20 to 30 nm. It will be appreciated from the following detailed discussion that the terms dielectric and buffer will be used interchangeably and are used to mean the same thing.
Other known prior art polarizing layers involve the use of Si.sub.3 N.sub.4 with a silver or gold overlay. Again, as in the case with SiO, the maximum polarization effectiveness will peak at precise values, thus making the manufacture of these devices very tedious, difficult and expensive because of the unusual control of the sputtering or deposition process required.
Other methods of inducing mode selective properties in an integrated optical strip waveguide include the use of biferingent crystal or liquid crystal overlays, proton exchange techniques either within or outside the waveguide, specific waveguide directions with respect to the crystal surface or anisotropic fabrication.
Each of these techniques include complications which are undesirable in the manufacture thereof. For instance, overlaying crystals requires the maintenance of pressure on the crystal, is expensive, bulky and requires accurately polished crystal surfaces. In addition, the unwanted mode is diverted from the waveguide, not attenuated, and may therefore be scattered back into the waveguide. The best reported extinction ratios has been 30 dB for these devices and the mechanical stability is also questionable.
Proton exchange is a relatively new technique for fabricating integrated optical devices. Unfortunately, there exists problems associated with both short and long-term drift with time, temperature, and applied electric field. Moreover, the waveguide dimensions required to achieve single mode strip waveguides are smaller than for titanium in-diffused guides and thus, hybrid systems incorporating both proton exchange and titanium in-diffused guides suffer problems of alignment and modal conversion at the boundary of the two guides.
Thus, in accordance with the present invention, the above-discussed problems are avoided and an effective method and device are provided which is easy to manufacture and which does not require the precise tolerance controls in the manufacture thereof, while still achieving high attenuation ratios along one mode with little or no attenuation along the other mode.